If this were a CW high school drama and not a post-apocalyptic adventure (sometimes it can be hard to tell them apart in this book), Peter would be the main villain. He's a bully motivated by his own selfish needs. We don't really have anything positive to say about him. If you're the type who thinks if we have nothing nice to say, we shouldn't say anything at all, then imagine a nice blank page here, with the peaceful sound of birds chirping in the background.
Okay, now that all the nice folks are gone, it's time to badmouth this jerk.
Early in the novel, he tries to steal the hard drive of simulation data from Tris's bedroom (sneaking into a girl's bedroom is bad enough). Later, he helps Jeanine escape. Hmm. Then he shocks everyone by helping Tris escape from Jeanine. The reason: he feels that he owes her. "The idea that I owed you anything made me sick" (36.67).
If anything, that makes him even more unpredictable and dangerous than Jeanine, who has a clear goal in mind. Peter's only motivation is himself.